1. Field of the Invention
This invention generally relates to gas-liquid contactors used in the removal of acidic gases, such as from utility and industrial flue gases. More particularly, this invention is directed to a wet flue gas desulfurization (FGD) process and apparatus that remove nitrogen oxides (NOx) from a flue gas following the removal of acidic gases such as sulfur dioxide.
2. Description of the Related Art
Acidic gases, including sulfur dioxide (SO2), hydrogen chloride (HCl) and hydrogen fluoride (HF), are known to be hazardous to the environment, and as a result their emission into the atmosphere is closely regulated by clean air statutes. For the removal of acidic gases from flue gases produced by utility and industrial plants, gas-liquid contactors and absorbers, or scrubbers, are widely employed. Scrubbers generally employ a liquid media that is brought into intimate contact with a flue gas to remove acidic gases by absorption. The process by which acidic gases are removed from flue gases in this manner is generally referred to as wet flue gas desulfurization (wet FGD).
The cleansing action produced by scrubbers is generally derived from the passage of a flue gas through a tower cocurrently or countercurrently to a descending liquid medium. Calcium-based slurries, sodium-based solutions and ammonia-based solutions are typical alkaline scrubbing media used in flue gas scrubbing operations. The cleansed gases are allowed to exit the tower, typically passing through a mist eliminator to atmosphere. The liquid medium and its absorbed gases are collected in a tank, typically at the bottom of the tower, where the absorbed gases are reacted to form byproducts that are useful or at least not harmful to the environment. While scrubbers utilizing calcium-based slurries generally perform satisfactorily, their operation results in the production of large quantities of wastes or gypsum, the latter having only nominal commercial value. In contrast, ammonia-based scrubbing processes have been used in the art to produce a more valuable ammonium sulfate fertilizer, as taught by U.S. Pat. Nos. 4,690,807 and 5,362,458, each of which are assigned to the assignee of the present invention. In these processes, the scrubbing solution is accumulated in a tank where the absorbed sulfur dioxide reacts with ammonia (NH3) to form ammonium sulfite ((NH4)2SO3) and ammonium bisulfite (NH4HSO3), which are oxidized in the presence of sufficient oxygen to form ammonium sulfate ((NH4)2SO4) and ammonium bisulfate (NH4HSO4), the latter of which reacts with ammonia to form additional ammonium sulfate. A portion of the ammonium sulfate solution and/or ammonium sulfate crystals that form in the solution can then be drawn off to yield the desired byproduct of this reaction.
Nitrogen oxides (NOx), which include nitric oxide (NO) and nitrogen dioxide (NO2), are also commonly found in flue gases produced by utility and industrial plants. The presence of NOx in the effluent of a scrubber is environmentally undesirable because of links to smog and ozone deterioration. Because nitric oxide readily oxidizes at room temperature (about 20xc2x0 C. to about 25xc2x0 C.) to form nitrogen dioxide, nitric oxide present in flue gases tends to form nitrogen dioxide which, depending on atmospheric conditions, can produce an unsightly yellow-brown plume. The elimination of NOx from flue gases has typically focused on the avoidance of forming nitrogen dioxide. For example, nitrogen gas (N2) is produced in processes involving NOx dissociation by selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR). As the primary component of air, the release of nitrogen gas into the atmosphere does not pose an environmental concern. However, an ongoing demand of desulfurization processes is the reduction of emissions. From an economics standpoint, it is also desirable for a desulfurization process to produce valuable byproducts. Electron beam and low NOx burners have also been proposed as methods for removing Nox. For example, irradiation with an electron beam has been used to convert NOx to ammonium nitrate (NH4NO3) in the presence of ammonia, as disclosed in U.S. Pat. Nos. 5,834,722 and 6,179,968. However, disadvantages of ion beam treatments include their relatively high cost and low efficiency.
The present invention provides an apparatus and process for removing acidic gases and NOx from flue gases produced by fuel combustion operations of the type carried out in utility and industrial plants. The process and apparatus of this invention intentionally convert NOx, and particularly nitric oxide, to nitrogen dioxide, which is then reacted to form a valuable byproduct. As such, the process and apparatus of this invention differs from prior processes in which the object was to convert NOx to nitrogen gas, which is then released into the atmosphere.
The process of this invention generally entails contacting a flue gas with a scrubbing medium, preferably an ammonium sulfate-containing scrubbing solution, to absorb acidic gases from the flue gas and produce an acidic gas-containing solution and an intermediate flue gas. According to the invention, after contact with the scrubbing medium, the intermediate flue gas is cooled to cause nitric oxide to be oxidized to form nitrogen dioxide, which is then absorbed from the intermediate flue gas to produce a NO2-containing solution and a more fully scrubbed flue gas. The nitrogen dioxide in the NO2-containing solution is then reacted with an aqueous ammonia solution to form ammonium nitrate (NH4NO3) as the valuable byproduct of the process. A flue gas scrubbing apparatus suitable for use with this invention comprises means for contacting the flue gas with the scrubbing medium to remove acidic gases and produce the intermediate flue gas, means for reducing the temperature of the intermediate flue gas to convert nitric oxide present in the intermediate flue gas to nitrogen dioxide, means for absorbing nitrogen dioxide from the intermediate flue gas to produce the NO2-containing solution and a scrubbed flue gas, and means for reacting the nitrogen dioxide in the NO2-containing solution with aqueous ammonia solution to produce ammonium nitrate.
According to the above, NOx present in a flue gas is intentionally converted to nitrogen dioxide, which is the species typically avoided in prior art processes because of environmental concerns and the visible plume produced by nitrogen dioxide. However, the present invention uses a combination of oxidation of nitric oxide to nitrogen dioxide, followed by a controlled reaction with an aqueous ammonia solution, the result of which does not reduce NOx to nitrogen gas but instead captures NOx to produce a valuable byproduct.
Other objects and advantages of this invention will be better appreciated from the following detailed description.